Acute kidney Injury (AKI), a common complication after cardiac surgery, has a high impact on clinical outcomes. The diagnosis of AKI by serum creatinine usually is delayed and occurs 2-3 days after the surgery. The failure of prior interventional studies in cardiac surgery to attenuate AKI may be attributed to the delays in the diagnosis of AKI. The TRIBE-AKI (Translational Research Investigating Biomarkers in Early Acute Kidney Injury) Clinical Consortium was established to accelerate the development of biomarkers. The consortium is a multi-disciplinary group and includes investigators from five major academic centers and who have expertise in pre-clinical, translational, epidemiologic, and health services research. In the present project, the consortium will study the following three biomarkers for early diagnosis of AKI: urine interleukin 18 (IL-18), neutrophil gelatinase associated lipocalin (NGAL), and cystatin C. The consortium will conduct a prospective multi-center observational cohort study of 1800 patients receiving cardiac surgery. Serial urine and serum samples will be collected from the participating patients, along with some clinical data during hospitalization. We will compare the timing of increases in biomarker levels with the clinical diagnosis of AKI and we will demonstrate the ability of the biomarkers for early diagnosis by 24-48 hours. Also, we will evaluate the hypothesis that compared with serum creatinine, cystatin C will improve pre-operative risk stratification, and urine IL-18 and NGAL levels will be better markers of post-operative AKI along with a potential to prognosticate its severity. The availability of new biomarkers to replace serum creatinine will allow for the early and accurate diagnosis of AKI. The ultimate findings of this study will pave the way for larger multi-center studies of these biomarkers in other clinical conditions and for interventional clinical trials to prevent or to treat AKI.